Studying the temperature dependence of the laser induced birefringence in azo dye doped polymer films Ezeddin Mohajerani * , Nahid Hosain Nataj Laser Research Institute, Shahid Beheshti University, Evin, Tehran, Iran Received 7 February 2006; accepted 15 June 2006 Available online 24 August 2006 Abstract In this article the effects of different parameters such as sample thickness, concentration of dye in the polymer and the laser pump power, on the temperature dependence of the laser-induced birefringence in azo dye doped polymer films were investigated. DR1- PMMA, DR13-PMMA and MR-PMMA samplers are studied. It is found that the peak temperature at which the induced birefringence is maximum varies depending on the mentioned parameters. Although we might get larger induced birefringence by varying parameters, however the stability of the signal by fluctuation in the temperature might be more important in most applications. In order to achieve more stability, the peak temperature should be around the working temperature. It is possible to tune the peak temperature to the work- ing temperature by optimizing the parameters such as concentration, thickness and pump power for each sample. Ó 2006 Elsevier B.V. All rights reserved. PACS: 42.40.Eq; 42.70.Jk; 42.70.Ln Keywords: Azo dye; Dye doped polymer; Laser induced birefringence; Anisotropic response 1. Introduction Polymers containing azo dyes have been paid great attention recently due to their unique optical properties [1]. Azo dye doped polymer films show great anisotropy under laser illumination due to trans–cis photo isomeriza- tion and the following photo reorientation [2,3]. Laser induced anisotropy in this class of material is the main mechanism that makes them attractive for different appli- cations [4–6]. Anisotropic response of this class of material depends on different parameters such as chromophore type, concentration of dye in polymer; pump beam power, tem- perature and sample thickness. Studying the effect of these parameters on the anisotropic response of the sample is crucial. The temperature dependence of photo-induced isomerization in dye doped polymers [7] and liquid crystals [8] have been broadly observed. However there are still more details to be investigated. In this paper, the effect of temperature on the induced birefringence in polymer films doped with three different azo dyes is reported. It is expectable that increasing the temperature more than a certain temperature causes a decrease in the induced birefringence, which is due to ther- mal fluctuations [9]. The temperature, at which the induced birefringence is maximum or simply the ‘peak tempera- ture’, is shown to depend on the above-mentioned param- eters. The effective parameters may be altered to optimize the induced birefringence. 2. Materials and sample preparation Thin films of polymethylmetacrylate (PMMA) doped with different guest chromophores are studied. Disperse red 1 (DR1), disperse red 13 (DR13) and methyl red (MR), all from Aldrich, were chosen. Fig. 1 shows the molecular structures of the three chromophores. Samples 0925-3467/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.06.013 * Corresponding author. Tel.: +98 912 1990359; fax: +98 21 22431775. E-mail addresses: e-mohajerani@sbu.ac.ir, ezeddinmohajerani@ yahoo.com (E. Mohajerani). www.elsevier.com/locate/optmat Optical Materials 29 (2007) 1408–1415